This invention relates to instruments and devices for measuring the size of liquid drops which are falling or otherwise moving through space.
It is often of interest to know the size of liquid drops falling or passing through a defined space. It may be of scientific interest, for example, to measure the size of raindrops under a variety of conditions, or one might wish to measure the size of drops produced by the atomization of a carburetor, the spray of an aerosol, whether it be for the spraying of a paint, lubricant or another material. Because of this interest in determining the size of liquid drops, various devices have been invented for this purpose. The particle analyzing apparatus described in U.S. Pat. No. 2,825,875 to Stubbs is an example. That invention uses a pencil-like probe 3, having an exposed spherical tip 5. The tip is in electrical contact with a sensing element 4 within probe 3 which, in turn, is connected to a source of constant potential. When an uncharged particle contacts the exposed tip 5, an electrical charge varying with the size of the particle is drawn from the sensing element 4 and momentarily reduces its potential. The negative pulse or signal thus created is detected by suitable circuitry which amplifies it and correlates particle size with pulse amplitude. The resulting data is thus the result of an indirect method of measurement. It might also be noted that the size of the sampling area in which drop sizes can be measured at any given moment is quite small. This is true even with the modified probe tips illustrated in FIGS. 4, 5 and 6 of the said patent. It can be appreciated that for laboratory purposes and for a variety of other applications, it may be desirable at times to be able to monitor a relatively larger sampling area. Furthermore, it is often desirable to obtain a direct measurement of drop size, rather than one which is dependent upon the magnitude of electrical effects related to drop size.
Devices such as those illustrated in the Stubb's patent tend to be expensive because they are relatively complicated and require the generation and use of relatively high voltages. A further disadvantage associated with such devices is that particles or drops being measured can accumulate on the sensing tip, adversely affecting their operation. Also, the distribution of drop sizes cannot be directly calculated from the output of such a device since drops of all sizes exceeding a predetermined minimum will trigger the recording circuitry. Rather, one must deduce the distribution of drop sizes by sequentially selecting drop size thresholds and noting the increase or decrease in drop indications produced for each incremental threshold change. Finally, the usefulness of such devices is over limited by the fact that only completely uncharged particles or drops can be accurately detected and measured with them.
Another approach to measuring particle or liquid drop size is illustrated by U.S. Pat. No. 4,205,384. The invention described therein employs a sensor device such as a heat image camera, which detects the physical characteristics of particulate matter in the form of measured values representing the size and/or mass and/or heat content or possibly other features thereof. While such an approach is undoubtedly suitable for applications of the type described in that patent, such apparatus does not offer the precision and the advantages associated with a direct measurement system.
The present invention centers in a novel sensing element comprised of a plurality of conductive wires held in a spaced-apart parallel array at a precisely measured distance from one another. Even numbered wires are maintained at ground potential while the odd numbered wires are maintained at a positive voltage. Whenever adjacent wires are connected by a falling electrically conductive liquid drop, the potential of the grounded wire will rise while that for the positive wire falls. Each wire is connected to circuitry which will react to the change in potential by providing an indication of such change. The circuitry enables one to count the number of adjacent wires whose potential has changed and to thereby mathematically determine the size of the drop connecting them. The accuracy of the drop size determination thus made is dependent only upon the physical limitations involved in the spacing of the wires and their diameters.
An invention of the type indicated offers the advantage of direct measurement, thereby avoiding errors characteristic of indirect measurements. Moreover, the invention is simple in design and relatively inexpensive to produce. It is capable of monitoring a relatively large sampling area which may vary in size depending upon the purpose of the sampling and the anticipated frequency of drops whose sizes are to be determined. Furthermore, it is capable of detecting and measuring either charged or uncharged drops.